Studies on the genetic transmission of endogenous murine retroviruses have led to the chromosomal mapping of more than 10 distinct loci. Most recently, genetic crosses have been used to position 2 of the 3 mouse mammary tumor proviral loci carried by BALB/c mice. Analysis of additional laboratory strains and various feral mouse populations have been undertaken to describe the stability and wild mouse origin of specific loci and to identify additional genetic factors that alter in vivo virus expression and susceptibility to exogenous virus. These studies have shown that inbred and wild mice carry loci for induction of zenotropic murine leukemia virus (MuLV) other than the Bxv-1 locus carried by at least 6 of the older inbred strains. Such loci were identified in MA/MY and NZB/B1N mice and in a partially inbred strain of fferal Japanese mice, M.m. molossinus. Studies on the endogenous ecotropic and zenotropic MuLV related proviral sequences found in a variety of feral mouse populations have shown that almost all lack ecotropic-MuLV related sequences and many lack xenotropic MuLV related sequences. However, MuLVs with ecotropic host range have been isolated from M. hortulanus, M.m. castaneus and M.m. molossinus. Southern blot hybridization data indicate that these viruses differ from ecotropic MuLVs of laboratory mice in hybridization properties and internal restriction maps. Cells of most wild mice also differ from inbred mice in susceptibility to exogenous infection. Genetic crosses show that susceptibility to xenotropic MuLV is controlled by a single chromosome 1 locus, Sxv. These studies also show that the feral mice M.m. praetextus and M. spretus also lack Fv-1 restriction. Genetic crosses indicate that these mice carry novel alleles at Fv-1.